Low loss, noise filtering multiplexer/demultiplexer for reconfigurable OADMs

ABSTRACT

Agile OADM structures having a range of tradeoffs between costs and flexibility are disclosed. In certain implementations, cyclic AWGs (arrayed waveguide gratings) are employed. Excellent optical performance is achieved along with relatively low initial and upgrade costs. An economically optimal level of network flexibility may thus be achieved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/630,582filed Jul. 29, 2003, which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

The present invention relates to optical networking and moreparticularly to systems and methods for Wavelength Division Multiplexing(WDM) communications.

Impressive strides have been made in the development of WDMcommunication links. Modern WDM communication links can carry a largenumber of wavelengths each modulated by a very high data rate signal.Also, the distance over which WDM signals can be transmitted withoutregeneration by way of optical-electrical-optical conversion has beenincreased. Furthermore, the distance between purely opticalamplification sites along such links has increased.

Telecommunication service providers are, however, also very interestedin the economic performance of WDM communication links. Revenue garneredby such links may be low initially and then grow over time as trafficincreases. To allow profitable operation even before the maturation oftraffic growth, it is desirable to install capacity in stages to theextent that technology allows. Rather than initially installing all ofthe optical components and systems necessary for a full capacity linkimmediately it is preferable to set up a modularized architecture wherelower cost partial initial deployments are possible.

To support this type of modular installation and upgrade path, it isimportant to provide an agile optical add-drop multiplexer (OADM)architecture. An OADM adds and/or drops wavelengths of a WDM signal. Inthe typical traffic growth scenario, the number of added/droppedwavelengths will grow over time. A conventional WDM that fulfills themaximum expected add/drop capacity requirement will be very costlyrelative to initial revenues.

There are known WDM structures that provide the needed flexibility.Although OADM flexibility postpones certain costs into the future,flexibility itself may also carry a cost due to the types of componentsthat are used. It is thus necessary to find the right trade-off betweenrequired flexibility in installation plus upgrade costs.

Flexible OADM structures are known. One type of known flexible OADMstructure provides automatic reconfigurability using, e.g., opticalswitches. This may be referred to as Reconfigurable OADM (R-OADMs).Another type of flexible OADM is manually reconfigurable using e.g.,fiber patch-cords and wavelength selective devices. These manuallyreconfigurable OADMs can be referred to as Flexible OADMs (F-OADMs).Technologies are available currently for implementing both R-OADMs andF-OADMs. For example, it is known to implement an F-OADM using amultiplexer arrayed waveguide grating (AWG) having a number of inputports corresponding to the maximum number of wavelengths to be added anda demultiplexer AWG having a number of output ports corresponding to themaximum number of wavelengths to be dropped.

The flexibility of the known OADM architectures comes at high initialcost and thus does not support the desired business model. Furthermore,many of the current agile architectures suffer from poor opticalperformance, e.g., high insertion loss on added/dropped wavelengthsand/or injection of additional noise on added wavelengths. Existingfixed OADM structures are cost and performance effective only for lowcounts of channels to be added or dropped and only where trafficreconfiguration or future growth is not an issue. What is needed areOADM structures that provide reconfigurability to accommodate futuregrowth and changes in traffic, that have good optical performance, andthat have relatively low initial and upgrade costs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a top-level representation of OADM functionality.

FIG. 2 depicts a split-and-select architecture for an OADM.

FIG. 3 depicts an add module according to a first embodiment of thepresent invention.

FIG. 4 depicts an add module according to a second embodiment of thepresent invention.

FIG. 5 depicts an add module according to a third embodiment of thepresent invention.

FIG. 6 depicts an add module according to a fourth embodiment of thepresent invention.

FIG. 7 depicts an add module according to a fifth embodiment of thepresent invention.

FIG. 8 depicts an add module according to a sixth embodiment of thepresent invention.

FIG. 9 depicts a drop module according to a first embodiment of thepresent invention.

FIG. 10 depicts a drop module according to a second embodiment of thepresent invention.

FIG. 11 depicts a drop module according to a third embodiment of thepresent invention.

FIG. 12 depicts a drop module according to a fourth embodiment of thepresent invention.

FIG. 13 depicts how an add module may be integrated to provide a lowcost, high performance upgrade path according to one embodiment of thepresent invention.

FIG. 14 depicts how an add module may be integrated to provide a lowcost, high performance upgrade path according to an alternativeembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a representative OADM architecture to which embodimentsof the present invention may be applied. An OADM 100 adds and dropswavelengths of a bi-directional WDM link. The bidirectional linkconsists of two unidirectional links flowing in opposite directions. Oneof the unidirectional links 102 is said to flow from west to east. Theother unidirectional link 104 can be said to flow from east to west. Awest side add/drop module 106 adds wavelengths to the signal flowing tothe west and drops wavelengths from the signal flowing from the west.Similarly, an east side add/drop module 108 adds wavelengths to thesignal flowing to the east and drops wavelengths from the signal flowingfrom the east.

Thus it can be seen that an OADM is an optical network element havingtwo bi-directional line interfaces and two bi-directional client (ortributary) interfaces. The OADM is able to extract/insert a subset ofwavelengths from/to the incoming/outgoing WDM wavelength set and routethese wavelengths to/from the client drop/add interfaces.

The number of individual wavelength client interfaces define theadd/drop capacity which can be described as a ratio between the numberof wavelengths that can be added or dropped and the total number ofwavelengths in the WDM grid. For example, an OADM able to drop up to 16wavelengths out of a total of 32 has an add/drop capacity of 50%.

To quantify OADM flexibility, it is useful to introduce a parameter thatmeasures the number of supported possible configurations. Assuming anadd/drop capacity of 100%, an ideal agile OADM would be able to add/dropany combination of the N wavelengths of the WDM grid. For this idealcase, any particular wavelength may be added/dropped or simply passedthrough. Each wavelength thus may be understood to have an associatedbit (“1” or “0”) to describe the add/drop or pass-through state. Thenumber of possible states for this ideal agile OADM is therefore 2N.However, in order to optimize parameters such as cost and opticalperformance, it may be desirable to provide an agile OADM architecturewith less flexibility where certain combinations of states for all thewavelengths are not achievable. The ratio between the number ofsupported states in a particular agile OADM architecture and the idealcase of 2N states is a useful measure of flexibility.

FIG. 2 depicts further details of add/drop east side module 108 thatemploys a so-called “split and select” architecture. For the WDM signalflowing from east to west, a splitter 202 taps off a portion of that WDMsignal and forwards it to a drop module 204. Drop module 204 thenseparates out the various wavelengths to be dropped. Wavelengths to beadded to the optical signal flowing from west to east are combined in anadd module 206. The combined add signal is then mixed into the west toeast WDM signal by a coupler 208. Embodiments of the present inventionprovide improved implementations of drop module 204 and add module 206.

Add Module

FIG. 3 shows an add section structure that provides 100% capacity (and100% flexibility). In the examples described herein, the number ofwavelengths in the grid is N=32. In FIG. 3, within add module 206 thereis a cyclic arrayed waveguide grating (AWG) 302 that combineswavelengths at P input ports into one output port. Each input port ofcyclic AWG 302 accepts N/P wavelengths spaced apart at P times thesystem's WDM grid spacing. (It will be appreciated that many of the“accepted” wavelengths may not actually be present.) In the example ofFIG. 3, a 1:8 cyclic AWG is used for a 32 wavelength grid. In this caseeach of the P input ports carries four wavelengths. If the overall gridis 100 GHz, each input port grid has a spacing of 800 GHz. Cyclic AWG302 is wavelength-selective in that at any particular input port thewavelengths other than the accepted ones are rejected. This provides ahighly beneficial noise filtering effect not found in previous addstructure implementations that combine numerous wavelengths withoutfiltering.

A description of the details of cyclic AWG technology may be found inKaneko, et al., “Design and Applications of Silica-based PlanarLightwave Circuits,” IEEE Journal of Selected Topics in QuantumElectronics, Vol. 5, No. 5, September/October 1999, the contents ofwhich are herein incorporated by reference in their entirety for allpurposes. Although the present invention is described with reference tothe use of cyclic AWGs for multiplexing and demultiplexing, one may alsosubstitute, e.g., optical interleavers and deinterleavers or othersuitable devices.

Although cyclic AWG 302 accepts four wavelengths at each of 8 inputs,all of the wavelengths may not be present at all of the inputs. In fact,it would be very typical that upon initial installation only a fewwavelengths are utilized while others are added later. The design ofFIG. 3 provides 100% capacity and flexibility by providing separateinputs for each of the 32 wavelengths.

Thus, four wavelengths are combined to produce one input to cyclic AWG302. Each wavelength is passed through an optional variable opticalattenuator (VOA) 304 that maintains polarization. As shown by thehorizontal and vertical arrows, each pair of wavelengths is preferablyinput with linear orthogonal polarization states. This allowscombination of the wavelength pairs to be performed by polarization beamcombiners 306. The polarization beam combiners may employ, e.g., InPtechnology, fused fiber technology, etc. The polarization beam combiners306 introduce relatively low insertion loss, e.g., approximately 0.3 dB,but reduce noise level by causing the noise from each channel to addincoherently. In an alternative embodiment, polarization beam combiner306 incorporates a quarter waveplate 310 that rotates the linearpolarization state of one input by 90 degrees. In this way all of theinput wavelengths may have the same linear polarization state,simplifying configuration.

A bandpass thin film filter (BP-TFF) 308 combines the outputs of PBCs306. BP-TFF 308 is a type of interferential filter that separates orcombines two adjacent multi-wavelength subbands. The output of eachBP-TFF 308 is fed to one of the P input ports of cyclic AWG 302. Each ofthe P input ports to cyclic AWG 302 may have a similar structure forcombining four wavelengths. Also, it will be seen that the use of thecyclic AWG 302, the polarization beam combiners 306, and the BP-TFF 308greatly reduce the introduction of optical noise while adding minimalinsertion loss.

FIG. 4 depicts an alternative add module having 50% add capacity and 5%add flexibility according to one embodiment of the present invention. InFIG. 4, there are two available inputs for each of the P ports of cyclicAWG 302. Thus one may optionally introduce two of the four possiblewavelengths for each of the P input ports. A 50/50 optical coupler 402combines the two input wavelengths. Both wavelengths each may passthrough an optional VOA 404. There is 50% capacity since 16 of 32channels may be added. The use of a coupler introduces some degree ofinsertion loss, typically around 3.5 dB. Also since there is no pre-AWGfiltering or use of a polarization beam combiner, there is someintroduction of out-of-channel noise in the process of combiningwavelengths prior to input and a 3 dB optical signal to noise ratio(OSNR) impairment (compared to what would be achieved using a PBC or notinput coupling to the cyclic AWG). The cost of this approach is howeverlower than in the 100% capacity example of FIG. 3.

FIG. 5 depicts a variation of the add module shown in FIG. 4 accordingto one embodiment of the present invention. TFFs 406 are single-channelbandpass filters introduced on each wavelength input. This increasesinsertion loss but filters the noise introduced on each input so thatthere is 0 dB OSNR impairment. Each TFF 406 is centered at theparticular input wavelength for that input.

FIG. 6 depicts another variation on the add module of FIG. 4 accordingto one embodiment of the present invention. In FIG. 6, a polarizationbeam combiner 602 combines the two input wavelengths for each input portof cyclic AWG 302. The arrangement of FIG. 6 assumes that the two inputwavelengths are polarized orthogonally to one another. This can beaccomplished by specifying the corresponding transmitters to outputorthogonally polarized signals or, alternatively by specifying that theyshare the same polarization and that a quarter waveplate 604 beintroduced as discussed with reference to FIG. 3. The arrangement ofFIG. 6 provides very low insertion loss and 0 dB OSNR impairment due tothe operation of PBC 602. For the arrangement of FIG. 6, capacity is 50%and flexibility is 5%.

FIG. 7 depicts a further alternative add structure that can provide avariable degree of flexibility according to one embodiment of thepresent invention. Here each input port of cyclic AWG 302 can have two,three, or four associated wavelength inputs. Thin film filters (TFFs)702, 704, and 706 are cascaded together with TFFs (single-channelbandpass filters) 704 and 706 being optionally installed. Each TFF hastwo inputs and a single output. If only TFF 702 is present at each inputport then two wavelengths may be input to a single AWG port and there isthus a capacity of 50% and a flexibility of 5%. If TFF 704 is added ateach input port, then three wavelengths may be introduced and there is acapacity of 75% and a flexibility of 60%. The additional inclusion ofTFF 706 then provides 100% capacity and flexibility. It will beappreciated that other levels of flexibility and capacity may beachieved by using disparate numbers of TFFs at each input port. A VOA708 is optionally installed at the wavelength input of TFF 702.Insertion loss for the added wavelengths is relatively low and there isno OSNR impairment.

FIG. 8 depicts an alternative add structure having 25% capacity and0.0091% flexibility according to one embodiment of the presentinvention. One of four wavelengths may be input to each of the inputports of cyclic AWG 302. An optional VOA 802 is included for each inputport. The added wavelengths experience very low insertion loss and thereis no impairment of OSNR. Although this implementation provides lessflexibility, component cost is relatively low.

Drop Module

FIG. 9 depicts a drop structure architecture according to one embodimentof the present invention. Here, a cyclic AWG 902 is used as ademultiplexer. The input to cyclic AWG 902 is a tapped-off portion ofthe west-to-east signal. The input to cyclic AWG 902 may optionally passthrough a VOA 904. Each output port of AWG 902 carries N/P wavelengthsspaced at P times the overall grid spacing. Here N=32 and P=8. CyclicAWG 902 both separates and filters the wavelengths from the single inputport to the P output ports. In the depicted example, the wavelength gridhas 32 wavelengths. Depending on traffic demands anywhere from 1 to 32of these wavelengths may actually be operational and present at theinput to cyclic AWG 902. Each output port presents from one to four ofwavelengths depending on which wavelengths are actually operational.Each output port may have an associated divider 906 which furtherseparates the wavelengths. As will be shown by way of example, there aremany possible implementations of divider 906.

In FIG. 10, divider 906 is implemented as a 50/50 optical splitter 1002followed by two parallel TFFs 1004. Each TFF 1004 selects a singlewavelength. Two of the four potentially available wavelengths may bereceived for each of the P output ports of cyclic AWG 902. Thisrepresents a drop capacity of 50% and a drop flexibility of 5%. Splitter1002 introduces a certain amount of insertion loss.

FIG. 11 depicts an alternative divider structure for 906 that exploitscascaded TFFs. Depending on how many TFFs are installed at each outputport, one may implement 50%, 75%, or 100% capacity, corresponding to 5%,60%, and 100% flexibility, respectively. Other levels of capacity andflexibility may be arrived at by using varying numbers of TFFs on eachof the P output ports of cyclic AWG 902. The TFFs 1006, 1008, and 1010are connected in a cascade fashion, each device having a single inputand two outputs. The capacity and flexibility figures vary as TFFs areadded as in the add module embodiment of FIG. 7. This arrangementprovides very good insertion loss characteristics.

FIG. 12 depicts another alternative structure for divider 906. At eachof the P output ports, there is a connected BP-TFF 1202. Bandpass thinfilm filter 1202 has two outputs, each carrying half of the gridspectrum. The outputs are fed to TFFs (single-channel bandpass filters)1204 and 1206 respectively. TFFs 1204 and 1206 further filter andseparate into individual wavelengths. This design thus provides 100%capacity and flexibility. This design also provides very low insertionloss.

Modularity

The add and drop structure designs as discussed above can providebeneficial modularity on various levels. FIG. 13 shows how modularitycan be achieved using the add structure design shown in FIG. 6. Thedesign of FIG. 6 provides 50% capacity and 5% flexibility. Two of fourpossible wavelengths may be added via a single input port of cyclic AWG302. FIG. 13 thus depicts two optional transmitters 1302 and 1304 for asingle input port of AWG 302. Transmitters 1302 and 1304 have the sameoutput linear polarization state. The optical transmitters 1302 and 1304are selectably installed depending on current need. Any particular inputport may have 0, 1, or 2 transmitters installed. As demand increases,more transmitters are installed on the various input ports. Up to 16 ofthe available wavelengths may be populated with transmitters. Thisapproach can reduce cost and insertion losses.

FIG. 14 depicts how the modularity concepts of FIG. 13 can be extendedto cover components other than the transmitters. Here, on each inputport a single module, if present, holds the transmitters 1302 and 1304,the optional VOAs 404 and the polarization beam combiner 602. Each suchmodule provides the capability for two wavelengths. Wavelengths can thusbe added two at a time, as needed. In this way, the costs of not onlythe optical transmitters but also those of other components can bepostponed until justified by traffic demand. Furthermore, the opticalconnection to the input of AWG 302 is not polarization state dependent.It will be appreciated that many comparable modularity schemes can beimplemented on both the add structure and drop structure sides.

Compared to the prior art, embodiments of the present invention providelower optical insertion loss (and thus highly beneficial avoidance ofthe need to use per-channel optical amplifiers), better optical noiseperformance, and a cost-efficient family of architectures that allowinstallation and upgrade costs to be distributed over time to matchtraffic demands. Furthermore, many of the components used such as thecyclic AWG, polarization beam combiners, VOAs, and quarter waveplatesmay be integrated on the same optical chip. Also, compared to add anddrop structures that employ conventional non-cyclic AWGs, embodiments ofthe present invention provide lower cost and lower add structureinsertion loss for capacity levels up to 50%.

It is understood that the examples and embodiments that are describedherein are for illustrative purposes only and that various modificationsand changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication and scope of the appended claims and their full scope ofequivalents.

1. An apparatus comprising: an optical splitter configured to tap off aportion of a wavelength division multiplexed (WDM) signal in a WDMcommunication system employing a WDM grid having N wavelengths; a cyclicarrayed waveguide grating (AWG) configured to receive said tapped offportion of said WDM signal as an input and further configured to outputnon-overlapping interleaved sets of N/P wavelengths via each of P outputports, wherein P is greater than two; a 50/50 optical splittercomprising an input port and first and second output ports, wherein saidinput port is configured to be coupled to one of said P output ports andwherein the 50/50 optical splitter is configured to split one of saidnon-overlapping interleaved sets of N/P wavelengths received via saidone of said P output ports for output to each of said first and secondoutput ports, respectively; and a first and a second thin film filter(TFF) in parallel wherein the first TFF is configured to be coupled tosaid first output port of the 50/50 optical splitter and the second TFFis configured to be coupled to the second output port of the 50/50optical splitter, and wherein each of the first and second TFFs isconfigured to select one wavelength from said one of saidnon-overlapping interleaved sets of N/P wavelengths.
 2. A methodcomprising: tapping off a portion of a wavelength division multiplexed(WDM) signal in a WDM communication system employing a WDM grid having Nwavelengths; directing said tapped-off portion to a cyclic arrayedwaveguide grating (AWG) configured to receive said tapped off portion ofsaid WDM signal as an input and to output non-overlapping interleavedsets of N/P wavelengths via each of P output ports, wherein P is greaterthan two; receiving at an input of a 50/50 optical splitter one of saidnon-overlapping interleaved sets of N/P wavelengths from one of said Poutput ports; splitting via said 50/50 optical splitter said one of saidnon-overlapping interleaved sets of N/P wavelengths to first and secondoutput ports; and selecting in parallel one wavelength from said one ofsaid non-overlapping interleaved sets of N/P wavelengths at each of saidfirst and second output ports using a first and a second thin filmfilter (TFF) coupled to said first and second output ports,respectively.
 3. The apparatus of claim 1, and further comprising avariable optical attenuator coupled between said optical splitter andsaid cyclic AWG configured to variably optical attenuate said tapped offportion of said WDM signal.
 4. The method of claim 2, and furthercomprising optically attenuating said tapped off portion of said WDMsignal.